This invention relates to enzyme electrodes and, more particularly, to enzyme electrodes having an electrical response arising from the oxidation of a product of the enzyme reaction.
Enzyme electrodes are amperometric sensors that incorporate an enzyme as a catalyzing element on a conductive electrode surface. In one form of enzyme electrode, an enzyme catalyzes the reaction of a selected material with oxygen to produce a reaction product and hydrogen peroxide. The hydrogen peroxide is then oxidized on a catalytic electrode surface to provide an electrical signal that is functionally related to the quantity of selected material in the sample being analyzed. Suitable catalytic materials include platinum (Pt) and other elements from the platinum group of the Periodic Table. By way of example, various body substances, such as glucose, urea, uric acid, triglycerides, amino acids, lactic acid, etc., react with appropriate enzymes, e.g., glucose oxidase (GO-ase), galactose oxidase, alcohol oxidase, lactic acid oxidase, etc., respectively, to generate hydrogen peroxide.
The usefulness of enzyme electrodes can be greatly increased if the sensitivity of the electrode is increased to enable small quantities of material to be detected and if the reaction time of the electrode can be decreased to enable relatively rapid changes in the material concentration to be detected. It is also desirable for the enzyme to be highly stable on the electrode so that measurements can be continued over long periods of time.
In one approach to improving the response of enzyme electrodes, GO-ase enzymes have been applied to the surface of a catalyzed carbon electrode structure such as conventionally used in gas reaction fuel cells. See, e.g., U.S. Pat. No. 4,979,145, issued Nov. 13, 1990, to Bennetto et al. As described in the '145 patent, conventional fuel cell electrode materials are cleaned, treated with an immobilizing solution, and then immersed in a solution containing the enzyme to immobilize an enzyme on the electrode surface. In one embodiment the enzyme is protected by the application of any overlying porous film. The Pt catalyst loading on the electrode was from 5-15% by weight of the total carbon. A substantial improvement in response time, sensitivity, and stability is reported over conventional enzyme electrodes.
A improvement in providing enzymes on a conductive surface is reported in U.S. Pat. No. 5,082,550, issued Jan. 21, 1992, to Rishpon et al. An enzyme, such as GO-ase, is mixed with a perfluorosulfonic polymer and cast onto a catalytic conductive surface, such as Pt. The perfluorosulfonic polymer is permeable to most fluids of interest while providing an insoluble biocompatible matrix for the enzyme and protecting the enzyme against bacterial degradation in both in-vivo and in-vitro environments. The perfluorosulfonic polymer also dissolves large quantities of oxygen to promote the formation of hydrogen peroxide in a stable manner, while accommodating small scale fluctuations in the level of oxygen of the surrounding medium. Thin films of the cast material are generally desired in order to enhance the sensitivity and speed of response of the resulting enzyme coating. However, one limitation of the enzyme-loaded film is that any hydrogen peroxide that is generated in the film must traverse the film to reach the catalytic electrode surface in order for the signal to be generated.
In view of the above enzyme sensors, it is an object of the present invention to minimize the distance for hydrogen peroxide to travel to reach a catalytic surface.
It is another object of the present invention to provide the enzyme in a matrix that provides access to a selected material to enable a rapid signal production in response to changes in the concentration of the selected material.
Yet another object of the present invention is to maintain a stable oxygen supply about the enzyme to promote the formation of hydrogen peroxide and maintain proportionality between the measured signal and the concentration of the material that is analyzed.
One other object of the present invention is to promote the formation of hydrogen peroxide adjacent an appropriate catalyst for oxidation that produces an electrical signal.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.